Like andrew, the premise is misleading: any worthwhile, in-memory database inverts the memory cache and RAM situation because the whole point is to be in memory. They all typically have a way of storing that data in secondary storage over time. Solo [1], for instance, can do 700,000 transactions a second on a 32-core machine with data regularly saved to the disk.
Now, the strategy they use is interesting. I'd just rather see an apples-to-apples comparison of it against DB's like FoundationDB, F1 RDBMS, and others doing high-performance w/ strong consistency. The more modern stuff, that is. MySQL and its performance are quite dated.